The present invention relates to an oscillator circuit for high frequencies and, in particular, to a crystal oscillator circuit in which changes in the oscillation frequency due to variations in the power voltage are prevented.
Crystal oscillator circuits usually have highly stable frequencies, so they are applied as frequency sources in different types of electronic appliances. There have recently been requests for a crystal oscillator circuit with a high oscillation frequency, for building optical communications systems. One of these utilizes an emitter coupled logic (ECL) circuit that operates at high speed, as an oscillation amplifier. A conventional type of crystal oscillator circuit is shown in FIGS. 7 to 9, with FIG. 7 showing the crystal oscillator circuit, FIG. 8 showing the internal ECL circuitry, and FIG. 9 showing a simplified oscillator circuit.
This conventional oscillator circuit is configured of a resonance circuit 1 and an oscillation amplifier 5a, as shown within the dotted line in FIG. 7. The resonance circuit 1 is formed of a crystal oscillator 3 made of quartz crystal and dividing capacitors 4a and 4b, where the crystal oscillator 3 is of AT cut or the like and functions as an inductor component. The dividing capacitors 4a and 4b are connected to the two ends of the crystal oscillator 3 and each is connected to ground.
The oscillation amplifier 5a has the abovementioned ECL structure, where the ECL circuit integrates a differential amplifier having two inputs A and B and two outputs C and D, of mutually opposite phases. As shown by way of example in FIG. 8, in which the interior of the ECL circuit is bounded by broken lines, the emitters of a first transistor Tr1 and a second transistor Tr2 are connected in common to ground. The collectors of the two transistors Tr1 and Tr2 are connected to a power source Vcc and the bases thereof are connected to the input terminals A and B, to which signals of opposite phases are input.
The oscillation amplifier 5a also has the output terminals C and D which obtain signals of opposite phases from the emitters of a third transistor Tr3 and a fourth transistor Tr4 that are connected to the first transistor Tr1 and the second transistor Tr2. Ordinarily, pull-down resistors 9a and 9b are connected externally to the output terminals C and D, respectively, as loads. These resistors have large resistances on the order of 150 to 200 Ω, for example, to prevent overheating due to excessive DC currents and to stabilize the operation.
In addition, the two ends of the crystal oscillator 3 are connected between the input B and output C, of mutually opposite phase, of the ECL for oscillation, as shown in FIG. 7. A bias resistor 7 and a bias capacitor 8 are connected on the ECL input side and a buffer amplifier 5b using ECL circuitry similar to that for oscillation is connected the output side thereof. Each of the oscillation amplifier 5a and the buffer amplifier 5b is driven by a voltage supplied from the power source Vcc. Note that if the oscillation portions only of the oscillation oscillator amplifier 5a are drawn simplified, the result would be as shown in FIG. 9 where it is clear that only the pull-down resistor 9a provided between one input B and output C has an effect on the oscillation-related components.
With the conventional crystal oscillator circuit, the oscillation amplifier (ECL) 2 works to feed back and amplify the resonance frequency of the resonance circuit 1 that is connected between one pair of input B and output C, to maintain the oscillation of a rectangular waveform. Note that the configuration is such that the other input A and output D achieve an input and output that are of opposite phase to the input B and the output C, due to the differential amplifier structure. Load is applied to the pull-down resistors 9a and 9b shown in FIG. 7 and the two outputs of opposite phase are amplified by the buffer amplifier 5b, to obtain two values of oscillation output. The oscillation frequencies in this case substantially match the resonance frequencies, but from the viewpoint of the crystal oscillator 3 they are determined by the load capacitances on the circuit side.
However, this conventional crystal oscillator circuit has a problem in that the oscillation frequency changes with variations in the power voltage Vcc. In other words, since an active element (in this case, the ECL circuit acting as the oscillation amplifier 5a of FIG. 7) has a characteristic that varies with the power voltage, the oscillation frequency also changes. In this case, the oscillation frequency varies upward as the power voltage Vcc rises, in other words, it varies upward as the current in the crystal oscillator 3 increases, so that the slope changes in accordance with the characteristic of the crystal oscillator 3 (see FIG. 7).
For that reason, a stabilizing circuit is usually inserted between the power source and the oscillator circuit, to stabilize the power voltage Vcc as shown, for example, in FIG. 11. However, the stabilizing circuit in such a case causes losses in the power voltage Vcc. Thus, a problem arises in that the output level drops, particularly when the circuit is driven at a low voltage such as below 3.3 V.
When ECL circuitry is used as the oscillation amplifier 5a, as shown in FIG. 7, the pull-down resistor 9a is attached externally and is connected to the output terminal C, as described previously. For that reason, it is considered to reduce the value of the pull-down resistor 9a so that the current to the crystal oscillator 3 is also reduced. However, excessive DC current in the pull-down resistor 9a could lead to overheating in this case, so it is not possible to make the value of the pull-down resistor 9a small in this manner. Note that since the ECL that is usually used is a generic circuit, the pull-down resistors 9a and 9b are attached externally to set the resistance as required.
In addition, frequency changes due to variations in the power voltage Vcc increase as the oscillation frequency increases. This is because the thickness of the crystal oscillator 3 (quartz crystal fragment) decreases as the vibration frequency thereof increases in correspondence with the oscillation frequency, and it becomes sensitive to the drive level (electrical field) based on the power voltage Vcc. For that reason, changes in the oscillation frequency due to variations in the power voltage Vcc become a problem when the oscillation frequency is of the 600 MHz band.
The present invention also relates to a frequency-switching oscillator, in particular to a frequency-switching crystal oscillator (hereinafter called a “frequency-switching oscillator”) having a simple circuit design and a small number of components.
A circuit diagram that illustrates a prior-art example of this type of frequency-switching oscillator is shown in FIG. 12.
As shown in FIG. 12, the conventional frequency-switching oscillator comprises a plurality of (such as two) crystal oscillators 41a and 41b that operate as inductor components, dividing capacitors 42a and 42b that form a resonance circuit 47 therewith, and an oscillation amplifier (transistor for oscillation) 43 that amplifies and feeds back the resonance frequency of that resonance circuit 47. The oscillation transistor 43 grounds the emitter side through a load resistor 44, by way of example. Note that in this case, the oscillation frequency is roughly dependent on the resonance frequency of the resonance circuit 47, but strictly speaking it is determined by serial equivalent capacitances on the circuit side as seen from the crystal oscillators 41a and 41b. 
Ordinarily, the base of the oscillation transistor 43 is connected to the connection point between the crystal oscillators 41a and 41b and the dividing capacitors 42a and 42b, the emitter of the transistor 43 is connected to the center-point of the dividing capacitors 42a and 42b, and the collector of the transistor 43 is connected to the power voltage Vcc, as shown in FIG. 12, so that an output Vout is obtained from the emitter, by way of example. In addition, a bias voltage is supplied to the base of the oscillation transistor 43 by dividing bias resistors 45a and 45b. An electronic switch 46 switches between the crystal oscillators 41a and 41b, for the 100 MHz band and the 600 MHz band, by way of example, to select the desired frequency. In this case, the electronic switch 46 is configured of a semiconductor switching element that is designed to select on or off in accordance with a 1 or 0 signal from the exterior, by way of example. (See FIG. 3 of Japanese Laid-Open Patent Publication No. 2002-359521, for example.)
However, with this conventional frequency-switching oscillator, the crystal oscillators 41a and 41b switch between two frequencies. For that reason, the crystal oscillators 41a and 41b and also the dividing capacitors 42a and 42b that form the resonance circuit 47 are required to have capacitances within ranges that satisfy one of these oscillation conditions. This causes further problems in that it is necessary to provide another oscillation amplifier and also the resonance circuit 47, increasing the number of components.